Alternative Strategies for Clonal Plant Reproduction©

Alternative Strategies for Clonal Plant Reproduction 269 Alternative Strategies for Clonal Plant Reproduction© Robert L. Geneve University of Kentucky, Department of Horticulture, Lexington, Kentucky 40546 U.S.A. Email: [email protected] INTRODUCTION Pick up any biology textbook and there will be a description of the typical sexual life cycle for plants. In higher plants, the life cycle starts with seed germination followed by vegetative growth leading to flower and gamete formation with the ultimate goal of creating genetically diverse offspring through seed production. The fern life cycle is more primitive but follows a similar progression from spore germination to the gametophytic generation leading to sexual union of gametes resulting in the leafy sporophyte, which in turn creates the spores. Interestingly, plants have evolved unique alternative life cycles that bypass typical seed production in favor of clonal reproduction systems. This may seem counter- intuitive because sexual reproduction should lead to greater genetic diversity in offspring compared to clonal plants. These sexual offspring should have a higher potential to adapt to new or changing environments, i.e., be more successful. How- ever, investing in clonal reproduction seems to increase the likelihood that a species can colonize specific environmental niches. It has been observed that many of the perennial species in a given ecosystem tend to combine both sexual and clonal forms of reproduction (Ellstrand and Roose, 1987). Additionally, unique clonal propagation systems tend to be more prevalent in plants adapted to extreme environments like arctic, xerophytic, and Mediterranean climates. Mini-Review Objectives. The objective of this mini-review is to describe some of the ways plants have evolved clonal reproduction strategies that allow unique plants to colonize extreme environments. CLONAL REPRODUCTION ADOPTED BY PLANTS There are two basic forms of clonal reproduction adopted by plants. Flowering plants and ferns can reproduce by modifications of vegetative structures (shoot, leaf, and root). Additionally, flowering plants can produce clonal seeds via apomixis. It is interesting that cycads and gymnosperms that are intermediate in their evolu- tion between ferns and flowering plants do not display alternative clonal propagation strategies. It is possible that morphological adaptations in cycads and gymnosperms did not permit the plasticity needed for clonal reproduction. Vegetative reproductive systems are incredibly unique, and their diversity is listed in Table 1 and illustrated in Figure 1. These reproductive structures can be aerial or subterranean modifications of stems, leaves, inflorescences, and roots. They range from basal stem modifications that slowly colonize local areas (like off- set production in geophytes) to the production of detachable vegetative plantlets (like aerial bulbils) that can be disseminated much like true seed. Unlike the structural anomalies obvious in vegetative clonal reproduction, there are no obvious outward signs that clones formed through apomixes differ from their sexual counterparts. Apomixis is the production of an embryo that bypasses the usual process of meiosis and fertilization (Hartmann et al., 2002). The genotype Table 1. Clonal propagation strategies as alternatives to sexual seed production. 270 1. Modified stems (basal) a. Bulbs (offsets) tulip Tulipa lily Lilium b. Tubers potato Solanum tuberosum wind flower Anemone c. Corms (cormels) crocus Crocus Liatris gayfeather Combined Proceedings International Plant Propagators’ Society, Volume 56, 2006 56, Volume Society, Propagators’ Plant International Proceedings Combined d. Rhizomes iris Iris bamboo Bambusa e. Stolons (runners) strawberry Fragaria red stem dogwood Cornus sericea (syn. C. stolonifera) 2. Modified stems (above ground) a. Arial bulbs (bulbils) European bittercress Cardamine bulbifera (syn. Dentaria bulbifera) lily Lilium monkey flower Mimulus gemmiparus b. Arial tubers (tubercles) meadow saxifrage Saxifraga granulata devil’s tongue Amorphophallus bulbifer hardy begonia Begonia grandis subsp. evansiana rosary vine Ceropegia linearis subsp. woodii winged yam Dioscorea alata 3. Plantlets on leaves mother of thousands Kalanchoe daigremontiana (syn. Bryophyllum daigremontiana) piggy-back plant Tolmiea menziesii hen and chick fern Asplenium bulbiferum walking fern Asplenium rhizophyllum (syn. Camptosorus rhizophyllus) bulblet fern Cystopteris bulbifera button fern Tectaria cicutaria 4. Plantlets on roots pawpaw Asimina triloba Reproduction Plant Clonal for Strategies Alternative blackberry Rubus 5. Plantlets on inflorescence century plant Agave americana wild garlic Allium vineale spiderplant Chlorophytum comosum orchard grass Dactylis glomerata walking iris Neomarica caerulea meadow grass Poa bulbosa alpine bistort Polygonum viviparum 6. Roots on stems English ivy Hedera helix figs Ficus 7. Apomixis a. Gametophytic grasses Poa, Pennisetum dandelion Taraxacum crabapple Malus b. Sporophytic (adventive embryony) orange Citrus Mangifera mango 271 272 Combined Proceedings International Plant Propagators’ Society, Volume 56, 2006 Figure 1. Examples of clonal reproduction systems. (A) Corm and cormel production in gladiola; (B) Aerial bulbils in lily; (C) Aerial tubers in hardy begonia; (D) Plantlets along the leaf margin in mother- of-thousands (Kalanchoe); (E) Aerial bulblets on the frond of bulblet fern; (F) Root suckers in pawpaw; (G) Aerial bulbils in the inflorescence of meadow grass; (H) Multiple apomictic embryos in citrus. of the embryo and resulting plant will be the same as the seed parent. Seed production via apomixis is clonal. Some species or individuals produce only apomictic embryos; however, the majority of apomictic species produce both apomictic and sexual embryos. Apomixis can be further divided into gametophytic versus sporophytic apomixis. In gametophytic apomixis, the megaspore mother cell fails to initiate or complete meiosis. However, the unreduced (2n) mother cell goes on to form an otherwise normal embryo sac. The egg cell divides to form an embryo but it was never fertilized by a male gamete. The result is clonal seed production. In sporophytic apomixis, the megaspore mother cell does complete meiosis and forms an egg sac with reduced (1n) female gametes. These are fertilized and form normal endosperm and a sexual embryo. However, in addition to this single sexual embryo multiple clonal embryos spontaneously develop from nucellar (2n) tissue surrounding the sexual embryo sac but still inside the developing seed (ovule). The seed usually contains one sexual embryo and multiple asexual embryos. This type of apomixis is a form of polyembryony and is termed adventive embryony. An obvious question for biologists to ponder is whether reliance on vegetative propagation with its inherent reduction in genetic diversity allows the flexibility for a species to adapt to a changing environment and avoid extinction. There are few cases where a species relies on vegetative propagation as its predominant means of propagation. For example, the Minnesota trout lily (Erythronium propullans) re- produces by means of a dropper (propagation bulb) produced at the end of a stolon. This general vegetative strategy allows trout lily species to colonize relatively large Alternative Strategies for Clonal Plant Reproduction 273 areas. However, the Minnesota trout lily appears incapable of significant sexual reproduction to supplement vegetative propagation. It only rarely produces seeds and these seeds are thought to be hybrids with white fawnlily (E. albidum) (Banks, 1980). The result is significantly less genetic diversity in Minnesota trout lily compared to white fawnlily colonies (Pleasants and Wendel, 1989). Currently, this genetic bottleneck leaves the Minnesota trout lily an endangered species found only in two southeastern Minnesota counties. In contrast to the Minnesota trout lily, there are numerous species that reproduce predominately by vegetative means, but still are capable of seed production. Genetic studies indicate that there is reduced genetic diversity found within local colonies of clonal species and that this diversity increases with geographic distance as the number of dominate clones also increases (Ellstrand and Roose, 1987). The mother plant (genet) can dispense both genetic (seedlings) and clonal progeny (ra- met). When the clonal progeny is very adaptive to a particular environment, it be- comes the dominant reproduction unit (Eriksson, 1997). An extreme example is in aspen (Populus tremuloides) populations that can have over 1000 clonal offspring (ramets) from a single dominant mother plant (genet) covering over 30 acres (Kem- perman and Barnes, 1976). However, even in species where clonal propagation is dominant, sexual progeny from the mother plant provides the genetic flexibility to allow the species to adapt to a changing environment. Sexual reproduction provides essential genetic drift in a species that leads to greater adaptive capacity observed in clonal populations. Some violet species may take the prize for combining the most genetic and clonal strategies for plant reproduction within an individual plant. Violets can produce two types of flowers. Chasmogamous flowers are produced in the spring or summer when pollinators are plentiful and active. Chasmogamous flowers open to permit cross-pollination between flowers and produce offspring (seeds) with generous genetic diversity. They also produce

Load more